Short Notes on Acids (Acids, Basis and Salts)

Acids

Substances with sour taste are regarded as avoids. Lemon juice, vinegar, grape fruit juice and spoilt milk etc. taste sour since they are acidic. Many substances can be identified as acids based on their taste but some fo the acids like sulphuric acid have very strong action on the skin which means that they are corrosive in nature. In such case it would be according to modern definition-

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An acid may be defined as a substance which release one or more H+ ions in aqueous solution. Acids are mostly obtained from natural sources. One the basis of their source avids are of two types –

  • Organic Acids: Acids which are present in animals and plants are known as organic acids. A list of commonly used acids along with their chemical formula and typical uses, is given below:

  • Mineral Acids: Acids which are obtained from rocks and minerals are called mineral acids.Some common examples of mineral acids are Hydrochloric Acid, Sulphuric Acid and Nitric Acid etc.

Note: In general MINERAL acids are STRONG acids while ORGANIC acids are WEAK acids.

Strong and Weak Acids

(i) Strong Acids : Acids which are completely ionised in water are known as strong acids.

(ii) Weak acids: Acids which are weakly ionised in water are known as weak acids.

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Properties Of Acid:

These are some  important properties of acid

  1. Acids have sour taste :  Acidic foods are believed to taste sour because of their hydrogen ion concentration.
  2. Acids turn Blue Litmus Red:  Litmus is a natural dye which  has a chemical structure that can react with the protons of an acidic solution this reaction results in a color change.
  3. Acid solution are electrolyte (they conduct electricity): Acids produce the hydrogen ion when dissolved in water ( ) while bases produce the hydroxide ion ( ). These ions, when in water, act as charge carriers and can hence conduct electricity. Stronger acids dissociate completely and produce more of the ions. For instance, hydrochloric acid, a strong acid, would dissociate more, and hence, conduct electricity more strongly, while acetic acid is a weak acid, and would conduct more weakly.
  4. Acids react with metals to form hydrogen gas:  Most of the acids react with metals to form salts and evolve hydrogen gas. This shows that hydrogen is common  to all acids.
    Click here to refer to Bases Section of Acid, Bases and Salts

Metals + acids —-> Salt + Hydrogen gas

Here, dilute Hydrochloric Acid reacts with Magnesium  to give Magnesium Chloride and hydrogen gas. This is the reason why we should not keep eatables like curd etc (which contain acid in them) in metal wares .

5.  Acids react with metal carbonates (and metal Hydrogencarbonates) to form carbon dioxide gas : When an acid reacts with a metal carbonate (or metal hydrogencarbonate) then a salt , carbon dioxide  gas and water is formed:

Metal carbonate + Acids ——> Salt + Carbon Dioxide + Water

When the evolved Carbon Dioxide gas is being passed through lime water (Calcium Hydroxide Solution ). It turns Lime water milky (It was a white precipitate of Calcium carbonate ) but when access of carbon dioxide is passed through the ppt will first dissolve and then change to clear solution due to the formation of Calcium Hydrogen Carbonate which is solluble in water.
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6. Acids reacts with bases (or alkalies) to form Salt and Water :

Acid + Base —–> Salt + Water

When and acid and bases react with each other, they neutralize each others effect and gives salt and water as a product that is why  this reaction is also known as Neutralization Reaction.

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7.  Acids reacts with metal oxides (Hydroxide):  Acids mostly carried out upon heating react with metal oxides to form salt and water. These reactions are

Metal Oxide + Acid —–> Salt + Water

8. Acids have corrosive nature : The mineral acids cause severe burns on the skin and attack and eat up material like cloth, wood, metal structure and stonework, So they are said to be corrosive in nature.

Acids are never stored in metal containers because they are gradually corrode and eat up the metal container.

Some strong bases like sodium hydroxide are also corrosive in nature and attack and destroy our  skin.

 USES OF MINERAL ACIDS IN INDUSTRY :

  1. Sulphuric acid is used as manufacturer of fertilizers, paints, synthetic fibers, detergent, explosives and car batteries.
  2. Nitric acid is used for making explosives such as TNT, dyes and plastic
  3. Hydrochloric acid is used for removing oxides films from steel objects and removing “scale” deposits from inside the boiler.

Indicators

There are 118 chemical  elements  known to us at present to know which of them is an acid, basis or salt,  we use indicator.  An Indicator is a dye which  changes color when it comes in contact with an acid or a base . It tells us whether the substance we are testing is an acid or a base by changing its color . Some common example of indicators are Litmus ( litmus is a natural indicator), Methyl Orange, Phenolphthalein( both methyl orange and phenolphthalein are synthetic indicators).

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There are two types of indicators :

  • which change their color when comes in contact with an acid or base. e.g. litmus, methyl orange, red cabbage, extract ,turmeric.

Litmus, Red cabbage , Turmeric are natural indicators because they are obtained or extracted from natural products. e.g. litmus is extracted from lichen.

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Methyl orange, Phenolphthalein are Chemical indicators because hey are prepared chemically.

  • Olfactory Indicators : they change their  characteristic smell or fragrance in acidic and basic solution.e.g. Vanilla, Onion.
Indicators Natural Color   Acid Bases
Litmus Purple   red Blue
Methyl Orange Orange Red yellow
Phenolphthalein Colorless Colorless pink
Red cabbage extract red Red Green
Vanilla  Pleasant smell smell remain same cannot detect smell
Onion Characteristic smell smell remain same cannot detect smell

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